The present application relates to a system for controlling an electrical device.
Referring to FIG. 1, an electrical system 10 includes a control panel 20 that utilizes a digital computer (or otherwise) to provide effective control of many associated electrical devices. The centralized control panel 20 determines the effects on the entire system 10, or a portion of the system 10, for enabling or disabling an electrical device 24. For example, such associated electrical devices may include motors, pumps, generators, fans, valves, generators, switches, lights, etc. One type of control panel 20 is generally known as a programmable logic controller, such as those sold by Allen Bradley. The control panel 20 typically provides a low voltage, such as 0-24 volts (approximately), to a pilot relay 32. The pilot relay 32 is attached to the exterior of the a contactor housing 26, normally within a pilot relay housing 30, which is typically further located within a motor control center. The pilot relay 32 is electrically interconnected to a contactor 22 which switches on (e.g., starts or otherwise energizes) and off (e.g., stops or otherwise de-energizes) a remotely located associated electrical device 24, such as a motor. The interconnection of the pilot relay 32 to the contactor 22 is typically by a high voltage (or current) connection, such as for example, 120 volts (or approximately). The contactor 22 may alternatively be any type of control device, such as for example, a relay, switch, or starter. The contactor 22 is electrically connected to the electrical device 24 by three power cables 28a, 28b, and 28c for a three-phase electrical device. One, two, or three power cables are used for different types of devices.
The system shown in FIG. 1 is the traditional system for interconnecting a pilot relay to a contactor and has several disadvantages. Initially, an opening is drilled or otherwise opened in the side of the contactor housing 26 and the pilot relay housing 30 is typically threaded onto the contactor housing. The 120 volt wires controlling the contactor 22 are “fished” through the opening in the side of the contactor housing 26 and interconnected to the pilot relay 32 within the pilot relay housing 30, which is difficult and time consuming if the available space is limited. The pilot relay housing 30 is secured, typically with a nut, to the contactor housing 26. The wire(s) from the digital control panel 20 are likewise connected to the pilot relay 32 for controlling the pilot relay 32.
The size of the pilot relay and accordingly the pilot relay housing tends to be rather large because of the relatively large size of the relay necessary to switch 120 volts to the contactor for control. Unfortunately, the relatively large size of the pilot relay housing requires space that may not be readily available, especially in a crowded motor control center. Over an extended time period because of the mechanical nature of a relay, especially in unclean environments, the pilot relay have a tendency to fail or otherwise make unreliable connections. For example, the contacts of the relay may corrode or otherwise the relay may provide surges in current which are unacceptable for reliable operation of the electrical device.